Different types of actuators have been used to achieve autofocus in miniature cameras. Those actuators achieve autofocus based on two main mechanisms: single lens motion or whole barrel motion. In the former, a lens of about 5 mg from within the lens stack is translated up to 100 microns, whereas in the latter, the entire lens barrel of almost 45 mg is translated up to 200 microns.
Voice Coil Motors (VCMs) are widely used to achieve autofocus in digital still cameras; however, when a VCM is used in miniature cameras it raises a number of issues. The drawbacks in using VCMs include high power consumption, slow autofocus speed, large size of the motor, and an undesired lens tilt of 0.2° which results in a deterioration of the image quality.
A recent technology to achieve autofocus in miniature cameras is MEMS electrostatic actuation. This actuation method offers low power consumption, high response speed, and small size actuators. Currently, two types of electrostatic actuators are being pursued to achieve autofocus in miniature cameras. U.S. Pat. Ser. Nos. 8,358,925 B2/8,004,780 B2/8,786,967 B2 disclose autofocus systems for miniature cameras that work based on the concept of electrostatic comb-drive actuators. These autofocus actuators have a number of drawbacks. First, the structure of these actuators is complex as it includes many mechanical parts such as: rotary comb drive actuators, inner hinge flexures, ball-in-socket snubbers, movable frame, outer hinge flexures, motion control torsional flexures, cantilever flexures, fixed frame, pivot axis, serpentine contact flexure, pseudo-kinematic mount, and a platform. Second, the electrode lay-out that those actuators utilize does not lead to generating a large force, thus they are not suitable for translating large mass loads of the order of tens of milligrams, meaning that they cannot be used to achieve autofocus based on the whole barrel motion mechanism. Third, the structure of the actuators is not stiff enough as dimensions of the flexure mechanical components are thin, thus integrating the lens within the central ring of the actuator is a delicate and difficult process. Fourth, the nature of the structure of the actuators requires two mechanical snubbers to be attached on both sides of the rotors to stop undesired motion of the rotors during sudden shocks. This can be clearly understood by realizing that the rotors that hold the lens can freely move in two directions, up and down, and the stators cannot be used as stoppers against the rotors motion. These drawbacks make the assembly process of the camera module parts (MEMS actuators, lens barrel, housing, snubbers, etc.) complex and expensive.
U.S. Pat. No. 8,711,495 discloses a MEMS autofocus mechanism that utilizes three translational vertical comb-drive actuators to achieve autofocus in miniature cameras. This invention addresses two main concerns associated with MEMS actuators that prevented them from being used commercially: failure to survive a drop test and complexity in the integration of the optical elements into the actuator. Although the invention presents a stiff flexure actuator platform and a method for easy integration of the lens within the actuator, it requires a high driving voltage to generate a sufficiently large force to move a mg size payload because of the limited area of the electrodes of the actuator. The actuator has also a limited out-of-plane translational stroke, as the maximum height (thickness) of the electrodes is 20 microns, which is not enough to achieve autofocus. There is no MEMS technology that is being used to achieve OIS in miniature camera modules.
There is a need for MEMS actuator solutions to achieve autofocus and OIS in miniature camera modules. These MEMS actuators and associated components that form the camera module need to be sturdy, inexpensive, and easy to assemble. That necessarily requires MEMS electrostatic actuators that are reliable and sufficiently stiff to resist sudden shocks, are easy to integrate within the camera modules, and are inexpensive to manufacture.